Comment # 3 on bug 1183611 from Stefan Hundhammer

Additional note: If you really, really want to prevent getting any particular
package installed, even indirectly by some other package's dependencies, set it
to "taboo" (in zypper: set a lock).

That might mean getting a lot of dependency problems that you may have to
resolve manually; it might also mean getting a lot of other packages not
installed (because they require the taboo package).

In your example, set "yast2-core" to "taboo" in the detailed package selection.
I am pretty sure that will work; but you will most likely get a lot less
packages.